1. Field of the Invention
The present invention relates to an operational amplifier fabricated by an LSI (Large Scale Integrated) circuit process where high and low voltage devices are included in combination. More particularly, the present invention relates to an operational amplifier adapted for receiving a low voltage input signal and outputting a high voltage output signal.
2. Description of the Background Art
The operational amplifier in need of a high voltage output is constituted entirely by high-voltage (HV) transistors, as disclosed for example in Japanese patent laid-open publication No. 41752/1998.
However, since high voltage transistors, i.e. high withstand voltage transistors, are lower in threshold voltage (Vt)-specific accuracy than low voltage transistors, i.e. low withstand voltage transistors, an operational amplifier simply constituted unanimously by high voltage transistors would require its differential transistors, for example, to be increased in size in order to raise the Vt-specific accuracy. This, in turn, affects the area of a semiconductor chip of the integrated circuit.